1. Field of the Invention
The present invention relates to an imaging lens which forms an image of an object on a solid-state image sensor such as a CCD sensor or a C-MOS sensor used in a compact image pickup device, and more particularly to an imaging lens which is built in image pickup devices mounted in increasingly compact and low-profile smartphones and mobile phones, PDAs (Personal Digital Assistants), game consoles, information terminals such as PCs, and home appliances with a camera function.
2. Description of the Related Art
In recent years, there has been a general tendency that most information terminals have a camera function. Also, home appliances with a camera function have been introduced into the market; for example, a user who is away from home can see in real time what is going on at home, through the camera mounted in a home appliance by telecommunication between the home appliance and his/her smartphone. It is thought that products which enhance consumer convenience by adding a camera function to an information terminal or home appliance will be increasingly developed in the future. In addition, the camera mounted in such a product is expected to not only provide high resolution to cope with an increase in the number of pixels but also be compact and low-profile and provide high brightness and a wide field of view. In particular, the imaging lens to be built in mobile terminals is strongly expected to be low-profile enough to be applicable to low-profile products.
However, in order to provide a low-profile imaging lens with a wide field of view and high brightness as described above, the following problem has to be addressed: it is difficult to correct aberrations in the peripheral area of the image and ensure high imaging performance throughout the image.
Conventionally, for example, the imaging lenses described in JP-A-2010-026434 (Patent Document 1) and JP-A-2011-085733 (Patent Document 2) are known as compact high-resolution imaging lenses.
Patent Document 1 discloses an imaging lens composed of five constituent lenses, which includes, in order from an object side, a positive first lens, a positive second lens, a negative third lens, a positive fourth lens, and a negative fifth lens, features compactness and high brightness with an F-value of about 2, and corrects various aberrations properly.
Patent Document 2 discloses an imaging lens which includes a first lens group including a first lens having a convex surface on an object side, a second lens group including a second lens having a concave surface on an image side, a third lens group including a meniscus third lens having a concave surface on the object side, a fourth lens group including a meniscus fourth lens having a concave surface on the object side, and a fifth lens group including a meniscus fifth lens having an aspheric surface with an inflection point on the object side. This imaging lens is intended to realize a compact lens system with high resolution.
The imaging lens described in Patent Document 1, composed of five constituent lenses, corrects various aberrations properly and provides high brightness with an F-value of about 2.0 to about 2.5; however, its total track length is longer than the diagonal length of the effective imaging plane of the image sensor, bringing about a disadvantage in making the imaging lens low-profile. Furthermore, if this lens configuration is designed to provide a wide field of view, it will be difficult to correct aberrations in the peripheral area of the image properly.
The imaging lens described in Patent Document 2 provides a lens system which is relatively low-profile and able to correct aberrations properly. However, in order to ensure brightness with an F-value of 2.8 or less and a field of view of 65 degrees or more, the problem of difficulty in correcting aberrations in the peripheral area of the image must be addressed.
As stated above, in the conventional art, it is difficult to provide a low-profile imaging lens with a wide field of view which features high brightness and high resolution.